The field of the present invention is apparatus for measuring the brightness of an object and the present invention is particularly related to determining the exposure time of a photomicrographic apparatus.
It is known with respect to such an apparatus to tap part of the beam from the object to the film for visual observation and it is further more known to use an additional part of the beam for exposure metering as disclosed in U.S. Pat. No. 3,851,949, the disclosure of which is incorporated herein.
Such exposure-metering can be carried out using several methods. For instance an intermediary lens may be used to project the exit pupil of the objective through a beam splitter into an auxiliary plane in which is located a photoelectric detector feeding an output signal corresponding to the object brightness to a display device, or, if automatic control is desired, directly to the shutter control. However, the object itself may be projected through a beam splitter onto a plane in which again a photoelectric detector is located.
Particularly as regards the last case, the measuring system may be such as to allow switching from measuring the entire object image (integrated measurement) to measuring only part of the image (detailed or spot measurement) and vice versa by the introduction of different stops. Because the detail or spot important to the image however is located at the center of the object image only in exceptional cases, the suggestion already has been advanced to render the stop used to localize the image detail spatially variable, whereby its small aperture is practically moved to any location of the image, so that only light from this detail or spot reaches the photo detector and the remainder of the object, i.e. of its image, remains masked by the stop. On the other hand it is obviously also possible to mount the stop in place permanently and to displace the object until the desired detail appears at the aperture of this stop. This is already known from U.S. Pat. No. 3,106,129, but, compared to the spatially variable measuring field stop, it is cumbersome and hence disadvantageous.
However, there are also drawbacks in using a spatially variable measuring field stop considering that a relatively large space is required for the displacement of the stop and that both the optical and mechanical construction costs of displacing it are large.